git: manifest: always use .repo/manifests/ for the include dir

Even if the manifest lives at .repo/manifests.xml, repo will use
.repo/manifests/ to process include directives.  Stop assuming that
.repo/manifests.xml is a symlink pointing to the actual manifest
dir that we can follow.

BUG=chromium:1055830
TEST=`./run_tests` pass

Change-Id: I449c0953277de3428c67ce8a816b8eb5371ee5c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2072618
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Commit-Queue: Sean Abraham <seanabraham@chromium.org>
Tested-by: Sean Abraham <seanabraham@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2075596
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed